U.S. patent number 6,377,051 [Application Number 09/454,907] was granted by the patent office on 2002-04-23 for relay test set using computer controlled voltage supply to stimulate both voltage and current transformers.
This patent grant is currently assigned to ABB Power T&D Company Inc.. Invention is credited to Gregory J. Grote, Richard E. Tyner.
United States Patent |
6,377,051 |
Tyner , et al. |
April 23, 2002 |
Relay test set using computer controlled voltage supply to
stimulate both voltage and current transformers
Abstract
A relay test set of reduced size and weight includes a small,
highly efficient voltage and current source. The test set
incorporates a Class D switching amplifier design that greatly
reduces the size of the power source needed to inject the correct
voltage and current into the secondary inputs of a protective
relay. To further reduce the size, weight and cost of the relay
test set, a single voltage amplifier is used to drive both voltage
and current secondary inputs of a protective relay. In so doing,
this test set then becomes both a voltage transformer and current
transformer simulator utilizing a single voltage power source at
reduced size, weight and cost. To make the power source variable
while maintaining accuracy, a microcomputer module is used to
monitor both current and voltage being injected into the protective
relay. This microcomputer may also automatically run the required
relay tests and route the voltage or current to the appropriate
inputs of the protective relay by direct control of a relay
matrix.
Inventors: |
Tyner; Richard E. (Florence,
SC), Grote; Gregory J. (Jefferson City, MO) |
Assignee: |
ABB Power T&D Company Inc.
(Raleigh, NC)
|
Family
ID: |
23806560 |
Appl.
No.: |
09/454,907 |
Filed: |
December 3, 1999 |
Current U.S.
Class: |
324/418;
324/415 |
Current CPC
Class: |
G01R
31/3278 (20130101) |
Current International
Class: |
G01R
31/327 (20060101); G01R 031/327 () |
Field of
Search: |
;324/415,416,418,771
;361/71,152 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Brown; Glenn W.
Attorney, Agent or Firm: Woodcock Washburn LLP
Claims
We claim:
1. A test set for driving inputs of a unit under test (UUT),
wherein the UUT comprises a low voltage protective relay,
comprising a voltage and current source that includes a single
voltage amplifier to drive both voltage and current inputs of the
protective relay; a microcomputer for monitoring both current and
voltage being injected into the protective relay; and a relay
matrix operatively coupled to the microcomputer and the protective
relay, wherein the microcomputer routes the voltage or current to
the appropriate inputs of the protective relay by direct control of
the relay matrix.
2. A test set as recited in claim 1, wherein the microcomputer
automatically runs required tests.
3. A test set as recited in claim 2, wherein the voltage and
current source includes a Class D switching amplifier that is part
of the single voltage amplifier.
4. A test set as recited in claim 1, wherein the voltage and
current source includes a Class D switching amplifier that is part
of the single voltage amplifier.
5. A test set as recited in claim 4, wherein the Class D switching
amplifier comprises circuitry that includes a pair of digital
potentiometers controlled by the microcomputer; a first function
generator that outputs a variable frequency sine wave; a second
function generator that outputs a fixed frequency triangle wave;
and means for generating a PWM output based on the sine and
triangle wave outputs of the first and second function generators,
wherein a first one of the pair of digital potentiometers is used
by the microcomputer to control the frequency of the PWM output and
a second one of the pair of digital potentiometers is used by the
microcomputer to control the amplitude of the PWM output.
6. A test set as recited in claim 5, wherein the Class D switching
amplifier further comprises means for converting the PWM output
into a Class D voltage supply output for injection into the UUT.
Description
FIELD OF THE INVENTION
The present invention relates generally to the fields of protective
relaying and test apparatus. More particularly, the invention
relates, but is not limited to, a relay test set that utilizes a
computer controlled voltage power supply to simulate secondary
voltages and currents for testing a low voltage trip relay.
BACKGROUND OF THE INVENTION
A problem addressed by the present invention is that most relay
test sets having the ability to inject a minimum of ten amps of
alternating current at fifty volts are large and expensive. These
large and expensive test sets also use separate supplies for the
current and voltage. The present invention incorporates the ability
to inject both high current and voltage into the secondary inputs
of a protective relay using a single computer controlled voltage
power supply. By using this design, the relay test set is made to
be both cost effective and portable. The secondary inputs of
protective relays are used to connect the metering current and
voltage transformers. By injecting the voltage and current into
these inputs, the test set becomes a simulator of both current and
voltage transformers.
To reduce both size and cost, such a test set requires a very
efficient power supply that does not dissipate large amounts of
heat. A unique aspect of the present invention is the use of a
microcomputer module to control a variable voltage Class D power
supply to vary both voltage and current as needed for the required
test. The microcomputer also monitors the voltage and current being
injected into the unit under test, UUT, to insure the proper levels
are attained.
SUMMARY OF THE INVENTION
The present invention accomplishes several tasks in the area of
protective relay testing. Typical relay test sets are large, heavy
and costly. To reduce the size and weight of the test set, a small,
highly efficient voltage and current source has been designed.
Linear power supplies are very inefficient and require large heat
sinks to dissipate heat. A presently preferred implementation of
this invention incorporates a Class D switching amplifier design
that greatly reduces the size of the power source needed to inject
the correct voltage and current into the secondary inputs of a
protective relay. To further reduce the size, weight and cost of
the relay test set, one may also use a single voltage amplifier to
drive both voltage and current secondary inputs of a protective
relay, rather than using a separate voltage and current source. In
so doing, this test set then becomes both a voltage transformer and
current transformer simulator utilizing a single voltage power
source at reduced size, weight and cost.
It is desirable for the power source to be variable while
maintaining accuracy. To accomplish these goals, a microcomputer
module may be incorporated to monitor both current and voltage
being injected into a protective relay. This microcomputer may also
automatically run the required relay tests and route the voltage or
current to the appropriate inputs of the protective relay by direct
control of a relay matrix.
Other features of the present invention are disclosed below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a relay test set in accordance with
the present invention.
FIG. 2 is a diagram of a computer control pulse width modulator
portion of the Class D amplifier in accordance with the present
invention.
FIG. 3 is a diagram of the waveforms of the pulse width modulator
circuitry.
FIG. 4 is a diagram of the driver circuit, output transistors and
low pass filter of the Class D amplifier utilizing computer shut
down of the amplifier.
FIG. 5 is a diagram showing the computer controlled routing circuit
via a relay matrix and the voltage and current sensor circuit.
FIG. 6 is a flowchart of test set software logic in accordance with
the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The presently preferred embodiment depicted in FIG. 1 includes a
microcomputer module 1 with digital inputs and outputs (digital
I/O), LCD and keyboard interface and an analog to digital converter
(ADC), as shown. The microcomputer 1 controls the output of a Class
D Voltage Supply 2. The voltage output of the Class D voltage
supply is monitored by using a voltage divider 3 to reduce its
signal level to a point at which it can be handled by a "True RMS
to DC Converter" 7 and is then input into the microcomputer's ADC
circuit. The output of the Class D voltage supply 2 is also
connected to a relay matrix circuit 4. This enables the
microcomputer to route the proper signal to the secondary inputs of
a unit under test (UUT) 5. In the case of current injection into
the UUT, the current is routed from the Class D voltage supply 2,
through the relay matrix 4 and into the UUT 5 and returns through a
current shunt 6 to the ground of the supply (the grounding of the
current shunt is depicted in FIG. 5--see R3). As with the voltage
divider, the current shunt 6 outputs a low level voltage, but
proportional to the current, into the True RMS to DC Converter 7
and into the microcomputer module's ADC.
FIG. 2 shows more detail of a pulse width modulator (PWM) circuit
2-1 of the Digitally Controlled Class D Voltage Supply 2. The PWM
design incorporates digital potentiometers 8 and 9, and two
precision function generator IC chips 10 and 13. A triangle wave
output at a fixed frequency of 22 kHz is achieved from the function
generator chip 13. The function generator chip 10 is a variable
frequency sine wave generator controlled by the microcomputer 1
through the use of digital potentiometer 8. The digital
potentiometers 8 and 9 are controlled via three control signals,
"Up/Down," "Clock" and "Chip Select" (CS). Via the microcomputer's
Digital I/O, the digital potentiometer is selected by the chip
select line. Then the direction is chosen by the Up/Down input. The
Clock signal is finally used to move the digital potentiometer to
its desired position. By using this type of digital control, the
microcomputer can set the frequency of the injected signal to the
UUT 5 (FIG. 1). Likewise, by use of the digital potentiometer 9,
the amplitude of the injected signals can also be computer
controlled. The digital potentiometer 9 is inserted into the
feedback circuit of the operational amplifier 11 to control the
gain of the sine wave signal generated by the function generator
chip 10. The output of the gain amplifier 11 and the function
generator 13 is then input into an operational amplifier configured
as an infinite gain comparator 12. The result is a pulse width
modulation output 13 as shown in FIG. 3 (Note that FIG. 3 shows
approximately 1/2 of the sine wave cycle as an example of the PWM
output.)
The pulse width modulator output 13 is now controlled and amplified
by additional circuitry 2-2 as shown in FIG. 4. The pulse width
modulation 13 is input into an IGBT transistor driver IC chip 14
with high and low transistor drive outputs. The microcomputer can
turn on and off the amplifier by use of the shut down input of the
driver chip. The high and low outputs of the driver chip are
inverted so that only one IGBT transistor, 15 or 16, is on at any
time. As the pulse width modulator input goes high, IGBT 15 is
turned on and IGBT 16 is turned off. This pulls the output of the
series connected IGBTs to 80 VDC and isolates the output from
ground. Likewise, as the pulse width modulator goes low, IGBT 16 is
turned on and IGBT 15 is turned off. This pulls the output to
ground and isolates the 80VDC. This has now amplified the pulse
width modulation from approximately 0 to 12 VDC output to a signal
that switches from 0 to 80 VDC. The amplified pulse width maintains
its switching frequency of 22 kHz while being modulated at the
variable 50 to 60 Hz. The high frequency is now to be filtered out
of the desired injection signal and this is achieved using a low
pass filter. This filter, made up of L1, L2, C1 and C2, is a 12 dB
/ octave low pass filter and removes the high frequency component
to better than -75 dB. The result is the frequency and amplitude
controlled alternating current signal.
FIG. 5 shows the configuration of the relay matrix 4 for connection
of the UUT and the monitored voltage and current feedback signals
to the microcomputer ADC circuit. The output of the Class D voltage
supply 2 is connected to the inputs of relays K1 through K7. The
normally open contact of relays K1, 2, 3 and 4 are connected to the
UUT current inputs, phase A, B, C, and neutral, respectively. The
normally open contact of relays K5, 6, and 7 are connected to the
UUT voltage inputs, phase A, B, and C, respectively. This relay
matrix allows the microcomputer to control where the single-phase
voltage supply is injected to the UUT. The voltage divider circuit,
R1 and R2, reduces the output voltage to a lower level as needed at
the input of the true RMS to DC converter 7. The output of the
current shunt, R3, is also a low level voltage signal proportional
to the current as needed by the true RMS to DC converter. Both of
these signals are fed through a single pole, double throw relay,
K8. This allows the computer to monitor the desired signal, voltage
or current. This completes a closed loop feedback system. The
microcomputer can now set the desired frequency and amplitude of a
voltage or current injected signal, monitor the voltage and
current, and correct the signal to the proper level through the
feedback circuit.
FIG. 6 depicts the software logic for the relay test system. As
shown, the logic begins by deciding whether a current or voltage
test is to be conducted. If a current test is to be conducted, the
logic continues along the path shown on the left hand side of FIG.
6. On the other hand, if a voltage test is to be conducted, the
logic continues along the path shown on the right hand side of FIG.
6.
If a current test is to be conducted, the logic decides whether a
threshold or timing delay test is to be conducted. If a threshold
test is to be conducted, the logic takes the left-most branch of
the flowchart. If a timing delay test is to be conducted, the logic
takes the center branch of the flowchart.
In the left-most branch (threshold test), the logic first sets the
voltage output to a low value. Next, the amplifier is energized.
Next, the logic decides whether the device has "picked up." If not,
the voltage level is increased and the logic again tests whether
the device has picked up. Once the device has picked up, the
current level is measured and recorded.
Referring to the center branch of the flowchart, for the timing
test, the logic first sets the voltage output to an estimated
target value. Next, the amplifier is energized, and then the
current level is measured. The logic next determines whether the
current is within tolerance. If not, the voltage level is adjusted
as necessary, and the current level is again measured as indicated.
Once the current is within tolerance, the timing test is
performed.
Referring to the right-most branch of the flowchart, for the
voltage test, the logic first sets the voltage output to a target
level. The amplifier is energized, and then the device is queried
for its voltage level. Next, the actual voltage level is read, and
the two results (the voltage level as determined from the query and
the actual voltage level) are compared and any difference is
reported.
The present invention is not limited to the particulars of the
presently preferred embodiment disclosed above. Those skilled in
the art will recognize that many variations and modifications of
the invention can be made within the true scope of the present
invention.
* * * * *